Increased use of consumer electronics such as cellular telephones, laptop computers and other portable devices, and the development of new technologies like electric vehicles (EV) has increased the demand for compact, durable, high capacity batteries. This demand is currently being filled by a variety of battery technologies including traditional lithium ion batteries. However, the metal packaging of traditional batteries makes them heavy, thick, prone to leakage and difficult to manufacture. New generations of solid-state batteries are emerging that allow the fabrication of consumer batteries in a wide variety of shapes and sizes that are thinner, safer and more environmentally friendly. However, state of the art, solid-state batteries have several shortcomings including relatively low values of ion conductivity.
Lithium polymer electrolytes have received considerable interest for use in solid-state batteries. These electrolyte systems are complex materials composed of amorphous and crystalline phases. It has been known since 1983 that the ion motion in polymer electrolytes occurs predominantly in the amorphous phase. Accordingly, the conventional approach to improving ionic conductivity has been to investigate conditions that either decrease the degree of crystallinity or increase the segmental motion of the polymer matrix. However, despite significant improvements, modern lithium-ion batteries employing polymer electrolytes are limited by lithium ion conductivities of order 10−6 S cm−1 at ambient temperatures. This level of conductivity is not sufficient for many consumer battery applications.
The 10−6 S cm−1 conductivity ceiling was overcome by true solid-state batteries developed by Duracell in the 1970s which used pressed aluminum oxide (Al2O3) powder and Li salt (Lil) as the electrolyte material. See, U.S. Pat. No. 4,397,924 issued to Rea on Aug. 9, 1983 (Rea '924). The solid alumina electrolyte provided two orders of magnitude greater conductivity than polymer electrolytes. The lithium ions travel across the surfaces of alumina particles by hoping from oxide oxygen to oxide oxygen on the amorphous surface. (Kluger K, Lohrengel M, Berichte Der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics, 95 (11): 1458-1461 NOV (1991)). However, this ion conduction only occurs when sufficient contact between adjacent alumina particles is both created and maintained. The Rea '924 patent overcame the first part of the contact problem by severely compressing the components at compressive strengths of order 100,000 psi. The result is a very dense solid-state electrolyte. However, overtime the ionic conductivity of the electrolyte decreased as the contact between particles degraded. This was especially true when the electrolyte was subjected to shock or other mechanical trauma. Because Rea relied on physical compression to create contact between alumina particles, very small changes in the contact between the alumina particles could have a profoundly negative effect on the ion conduction of the material. In fact, it appears that this technology was virtually abandoned because of this limitation.
Recently aluminum oxide (Al2O3) membranes were considered for use as battery materials by other researchers, however, the mechanism for lithium-ion conductivity of the membrane itself has neither been considered nor explored, nor has the modification and adjustment of the membrane. For example, U.S. Pat. No. 6,586,133 issued to Teeters et al., on Jul. 1, 2003 (Teeters '133) teaches a nano-battery or micro-battery produced by a process comprising: providing a membrane with a plurality of pores having diameters of 1 nm to 10 μm, filing said membrane with an electrolyte; and capping each filled pore with an electrode from about 1 nm to about 10 μm in diameter in communication with said electrolyte to form individual nano-batteries or micro-batteries. While Teeters '133 suggests the use of aluminum oxide membranes, it teaches the membranes solely as an innocuous, inactive, “jacket” for containing or housing nano or micro cells. The Teeters patent is directed solely to the creation of nano and micro batteries and never teaches or even suggests using an active membrane to enhance the ion conductivity of the electrolyte in a synergistic manner. For example, the preferred pore diameter range of Teeters' system (up to 10 microns) is much too large for meaningful ion conductivity enhancement by the metal oxide membrane itself. Furthermore, Teeters teaches the use of AAO membranes with low pore densities and porosities which are inadequate for producing effective active (highly conductive) membranes. Thus, the membrane pores of Teeters function as simple compartments for containing a stack of anode, electrolyte, and cathode materials to form a cell. Teeters also teaches that the anode and cathode material of the preferred embodiment are contained inside the pore of the AAO membrane. Teeters invention, can be fabricated equally well by employing a variety of materials having pores. The principle of Teeters is the miniturization of a battery cell using AAO as a micro-container, not as a material for enhancing the performance of the battery itself.
Mozalev, et al. teach a porous alumina membrane as the separator for macrobatteries. See, A. Mozalev, S. Magaino, H. Imai, Electrochimica Acta, 46, 2825 (2001). Their work suggested that alumina membranes mechanically suppress Li dendrite formation, thereby improving cycling efficiencies. However, they have not suggested or discussed the lithium-coordinating role that modified aluminum oxide membrane walls can play. The object of the Mozalev invention is to mitigate formation of dendrites by use of a hard material for a battery separator. Any hard, porous, material will serve the object of Mozalev's invention.
U.S. Pat. No. 6,705,152 issued to Routkevitch et al., discloses a type nano-structured ceramic platform for gas sensors. Routkevitch's sensors comprise micro-machined anodic aluminum oxide films having high density nano-scale pores, sensing materials deposited inside the self-organized network of nano-pores and at least one electrode deposited on the AAO. The gas permeable electrodes are deposited upon the AAO so to provide electronic conductivity without closing the pores to outside gases, so to enable gas sensing. The object of Routkevitch's invention is to make nano or micro-sensing devices for detecting various substances at trace levels. Routkevitch teaches sensor devices that are open systems. Thus, the sensing materials deposited inside the network of nano-pores and the electrodes are continuously exposed to gas and liquid molecules from the ambient environment. A sensor device with blocked, clogged, or covered nano-pores is a closed system, and is not capable of performing the functions of sensing.
A major breakthrough in the room-temperature conductivity of lithium polymer electrolytes would significantly impact the rechargeable consumer battery market, as well as the emerging electric vehicle (EV) arena. Despite more than 20 years of active industrial and academic investigation, the current level of conductivity for lithium polymer electrolytes is not sufficient for many battery applications and suggests that a radical new approach based on a better understanding of ion transport is required.